Near-field explosion

The method presented above is based on the similarity of the blast waves of pressure vessel bursts and high explosives. This similarity holds only at some distance from the explosion. In the near field, the peak overpressure and impulse from a pressure... 

Previous studies of Vapor Cloud Explosions (VCE) have used a correlation between the mass of a gas in the cloud and equivalent mass of TNT to predict explosion overpressures. This was always thought to give conservative results, but recent research evidence indicates that this approach is not accurate to natural gas and air mixtures. The TNT models do not correlate well in the areas near to the point of ignition, and generally over estimate the level of overpressures in the near field. Experiments on methane explosions in "unconfined" areas have indicated a maximum overpressure of 0.2 bar (2.9 psio). This overpressure then decays with distance Therefore newer computer models have been generated to better simulate the effects... 

Shock waves in the near and far fields usually result from condensed phase detonations, or from an extremely energetic vapor cloud explosion. Most vapor cloud deflagrations will give rise to pressure waves in the near field which may propagate as a shock wave, or shock-up, in the far field. 

Although a vapor box or containment may be effective for concentration reduction, it may increase the explosion hazards. Higher explosion peak overpressure may be realized in the near field because of cloud geometry and partial blockage (Melhem and Croce, 1994). 

The EPA guideline for evaluation of the far-field (distant) explosion hazards is based on a TNT [TriNitro Toluene] model. Evaluation of the near-field (within the vapor cloud, or near the explosion center) requires departure from the blast-pressure and blast-impulse curves for TNT or modification of the distance/quantity relationship [22]. 

The calculated equivalent amount of TNT energy can now be used to estimate shock wave effects. The analogy of the explosion of a container of pressurized gas to a condensed phase point source explosion of TNT is not appropriate in the near field since the vessel is not a point source. Prugh (1988) suggests a correction method using a virtual distance from an explosion center based on work by Baker et al. (1983) and Petes (1971). This method is described below. 

Special care should be taken particularly in the case of gas cloud explosions as the TNT method overpredicts near field effects and underpredicts far field effects. In some States the application of TNT equivalency is limited to overpressure values of 0.5 bar and other approaches are used for higher values, such as multienergy methods in which the separate effects from pressure and drag wind coming from different explosion cells are accounted for. However, the use of different TNT equivalencies in the near and far fields can overcome such a modelling deficiency. In general, TNT methods are considered suitable for greater distances from the source, for which the source mechanism is less important and such a simplified approach is more realistic and widely valid. 

The estimate of the explosion energy suffers from the same type of inconvenience of Brode s equation, and the analogy of the explosion of a pressurized gas vessel to a TNT one appears improper in the near field, since the vessel cannot be regarded as a point source. In this case, the correction method uses a virtual distance from the explosion center (Petes, 1971), to fictitiously move the explosion centre with respect to the surface of the expanding gas. The maximum overpressure of the shock wave, i.e. that at the contact surface between the initial expanding gas sphere and the air, is evaluated as (Baker et al., 1983 Prugh, 1988) ... 

Both methods cannot be safely extrapolated in the near field, and both require some adjustment procedure using Baker s method, a discontinuity will occur, not only at the scaled distanc /J = 2, limit for the near field zone, but also at Ji = 3.5, due to a multiplier factor to be used for cylindrical yessels, which change its value from 1.6 to 1.4 at = 3.5 (AIChE/CCPS, 2000). On the contrary, Prugh s method makes use of a virtual distance, to properly adjust the distance from the explosion centre, and no discontinuity is observed in the peak overpressure profiles. Nevertheless, in the near field, the estimates obtained applying both methods likely suffer from greater uncertainty on the other hand, it should be honestly admitted that, in the near field, peak overpressure values often... 

A principal parameter characterizing an explosion is the overpressure. Explosion effect modeling generally is based on TNT explosions to calculate the overpressure as a function of distance. Although the effect of a TNT explosion differs from that of a physical or a chemical explosion (particularly in the near-field), the TNT model is the most popular because a large data base exists for... 

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